1. Field of the Invention
The present invention relates to an optical data medium recording method which records a data signal by means of a laser beam or similar light source, and particularly to a method of obtaining optimum irradiation conditions according to changes in the recording medium and recording device.
2. Description of the Related Art
Technologies for writing or reading a data signal to or from, respectively, an optical data recording medium using a laser beam are already commonly applied in optical disk devices. In the discussion hereinbelow, the data recording medium shall therefore be referred to as an optical disk. These technologies are widely available today as compact discs (hereinafter CDs) and laser video discs, and in write-once optical filing and retrieval systems, Rewritable optical devices have just recently become widely available, though they are still not common.
One available read/write (R/W) system is the so-called phase change disk which utilizes the reversible state capability of certain materials which change between an amorphous and crystalline state or between different crystalline states. The recording thin-films used in these devices assume either an amorphous or distinct crystalline state, and can be switched between these states, by specific heating and cooling conditions induced by a laser beam. In addition, the optical constants (refractive index, extinction coefficient) of these materials change with the amorphous and crystalline states. A data signal is written to the phase-change disk by selectively forming these two states in the recording thin-film according to the data signal, and the signal is read from the disk using the resulting optical changes (transmissivity or reflectivity).
A light irradiation method as shown in FIG. 9 has been proposed as one way to obtain these two states. Described in the Japanese Journal of Applied Physics (Vol. 1.26 (1987), Supplement 26-4, p. 61), this method emits to the recording thin-film on the optical disk a laser beam (b), the power of which is modulated between two power levels, a peak power Pp and a bias power Pb (Pp&gt;Pb) according to the data signal (a). When d laser beam of this type is emitted to the recording thin-film, that part of the film which is radiated by the peak power Pp beam is set to the amorphous state, and that part radiated by the bias power Pb beam is set to the crystal state, regardless of whether the pre-radiation state was amorphous or crystalline. It is therefore possible to overwrite a single spot on the film.
Data is Written to the optical disk by laser beam irradiation 43 thus described. The power levels Pp and Pb which define the laser irradiation conditions are constant levels optimized according to the type of optical disk. However, the optimum irradiation conditions required for any given recording thin-film vary with the passage of time, even for optical disks manufactured under the same conditions, and with variations in the manufacturing conditions. In addition, the power of the laser beam actually reaching the recording thin-film of the optical disk will also vary with soiling of the optical disk surface, or with a change in the operating conditions or with a drop in the transmission efficiency of the optical system in the recording device.
Shown in FIG. 14 is the C/N irradiation power dependency as one example of the changes in an optical disk. The optical disk and irradiation conditions are the same as those described hereinbelow with reference to the first embodiment of the present invention, and only those points related to the changes in the optical disk are described here. Curve A represents the recording characteristics measured at an initial (clean) state, and curve 13 represents the recording characteristics measured after the same optical disk was soiled by exposure for a predetermined period to a dusty environment. In general, the laser power yielding the greatest C/N ratio may be considered to be the optimum conditions for optical disk recording. The results of evaluation during the optical disk manufacturing process correspond to the state represented by curve A where the optimum irradiation power is 10 mw. However, when a signal is recorded with a 10-mW laser power after exposure to the above environment, the C/N ratio drops to a low 30 dB, demonstrating an unacceptable irradiation condition. It should be noted that if the disk surface is similarly soiled an the drive operation changes as described above, the recording state will change even more.
Thus, even if a signal is recorded according to the optimum irradiation conditions measured during optical disk manufacture, the actual recording will differ according to the conditions prevailing when the recording is made. As a result, when the data signal is demodulated from the recording area, the signal may not be correctly demodulated or read errors may be generated.